Numerical computations of wind turbine wakes

University dissertation from Stockholm : KTH

Abstract: Numerical simulations using CFD methods are performed for wind turbine applications. The aim of the project is to get a better understanding of the wake behaviour, which is needed since today’s industrial design codes for wind power applications are based on the BEM (Blade Element Momentum) method. This method has been extended with a number of empirical corrections not based on physical flow features. The importance of accurate design models does also increase as the turbines become larger. Therefore, the research is today shifting toward a more fundamental approach, aiming at understanding basic aerodynamic mechanisms. The result from the CFD simulation is evaluated and special interest is given to the circulation and the position of vortices. From these evaluations, it will hopefully be possible to improve the engineering methods and base them, to a greater extent, on physical features instead of empirical corrections.The simulations are performed using the program ”EllipSys3D” developed at DTU (The Technical University of Denmark). The Actuator Line Method is used, where the blade is represented by a line instead of a large number of panels. The forces on that line are introduced by using tabulated aerodynamic coefficients. In this way, the computer resource is used more efficiently since the number of node points locally around the blade is decreased, and they can instead be concentrated in the wake behind the blades.An evaluation method to extract values of the circulation from the wake flow field is developed.The result shows agreement with classical theorems from Helmholtz, from which it follows that the wake tip vortex has the same circulation as the maximum value of the bound circulation on the blade.